A balun is used for converting an unbalanced signal, transmitting through an unbalanced transmission line, to a balanced signal, transmitting through a balanced transmission line and vice versa. The balun is also capable of performing impedance conversion. For example, in the case where an unbalanced signal is provided to an unbalanced terminal of the balun, a pair of balanced signals, having phases that are different by 180 degrees (reverse phase) and having the same amplitude, are output through a balanced terminal of the balun. Further, the balun may also be formed in inner interconnection layers within a semiconductor chip.
A balun that can be used for GHz is available which is fabricated by multilayer interconnect technology performed on a substrate.
FIGS. 7(A) to 7(C) show an example of a conventional balun. FIG. 7(A) is a plan view thereof, and FIGS. 7(B) and 7(C) are cross sectional views thereof taken at different angles. In FIGS. 7(A) to 7(C), X is the horizontal direction of the main surface of the substrate, Y is the longitudinal direction of the main surface of the substrate, and Z is the vertical direction relative to the main surface of the substrate.
A balun 61 includes a dielectric film 62, a ground interconnect film 63 partially formed on the surface of the dielectric film 62, a dielectric film 64 formed on the dielectric film 62 covering the ground interconnect film 63, and a balanced interconnect film 65 partially formed on the surface of the dielectric film 64. The balanced interconnect film 65 has an end 65a connected to a terminal Port 2, and an end 65b connected to a terminal Port 3.
The balun 61 further includes a dielectric film 66 formed on the dielectric film 64 covering the balanced interconnect film 65, and an unbalanced interconnect film 67 partially formed on the dielectric film 66. The unbalanced interconnect film 67 is placed opposite the balanced interconnect film 65 via the dielectric film 66, forming an electromagnetic coupling region 68.
The unbalanced interconnect film 67 has one end connected to a terminal Port 1 of the unbalanced line. The vicinity of the other end of the unbalanced interconnect film 67 is connected to the ground interconnect film 63 via a through hole (or a via hole) running through the third and second dielectric films 66 and 64.
With through holes (or via holes) 70 running through the dielectric film 66, the ends 65a and 65b of the balanced interconnect film 65 are connected to electrodes of an external capacitor 71.
With the balun 61 thus structured, a radio-frequency signal provided through the terminal Port 1 to the unbalanced interconnect film 67 is transmitted to the balanced interconnect film 65 by an electromagnetic coupling at the electromagnetic coupling region 68 where the unbalanced interconnect film 67 and the balanced interconnect film 65 are opposing each other. The transmitted radio-frequency signal is output through the terminals Port 2 and Port 3 of the balanced lines as balanced signals having 180 degrees phase shift.
In addition, with recent advance in miniaturization techniques, a radio-frequency circuit on a Si semiconductor substrate has been developed. However, there is a problem in that losses of passive elements, such as transmission lines or inductors, are large due to low resistance of the Si semiconductor substrate. In order to block the influences from the Si semiconductor substrate, a method is used which grounds interconnect layers below interconnect layers where the transmission lines and passive elements are formed.
In the case where the method is used for the balun, a common mode impedance Ze is small because the distance between the interconnect layers forming the balun and the ground is short. Thus, Ze/Zo that is the ratio of the common mode impedance Ze to differential impedance Zo is small. This is not preferable in a sense that a greater value of Ze/Zo is required to increase the bandwidth of the balun.
It is possible to increase the value of Ze/Zo by decreasing the differential impedance Zo between the unbalanced line and the balanced lines of the balun. In the case where the unbalanced line and the balanced lines of the balun are placed on the same plane, the space between the unbalanced line and the balanced lines are determined depending on the degree of precision of the lithography technique which forms interconnections. Thus, it is not possible to sufficiently narrow the space to an adjacent interconnect film, thereby imposing limitations in decreasing the differential impedance Zo.
The balun of the conventional example shown in FIGS. 7(A) to 7(C) is useful to overcome the limitations. More specifically, by forming the unbalanced interconnect film 67 and the balanced interconnect film 65 via the dielectric film 66 in the stacking direction, it is possible to narrow the space between the unbalanced interconnect film 67 and the balanced interconnect film 65 more than the case where the balun is formed on the same plane. Therefore, by increasing the degree of electromagnetic coupling of the unbalanced interconnect film 67 and the balanced interconnect film 65, the differential impedance Zo can be decreased, thereby increasing the bandwidth of the balun.